In electrostatic or electrophotographic image forming apparatus, such as monochrome and color laser printers and photocopiers, it is well known to pass imaged media through a fusing nip to fuse the image into the media. The fusing nip is typically created by biasing two rollers together. In some fusing systems both rollers are positively heated. In other systems one roller is positively heated (the heated roller) while the other is unheated (the pressure roller). Of course it will be appreciated that the "unheated" pressure roller is itself heated by contact with the heated roller. The side of the media containing the toner image is contacted by the heated roller to fuse the toner into the media. The fusing process thus utilizes a combination of elevated temperature and pressure within the fusing nip to accomplish the desired amount of image fusing.
One area in which the fusing process can present problems is duplex imaging or duplexing. Duplexing involves imaging on both sides of a sheet of media. In many duplexing systems a first toner image is formed and fused onto a first side of the media, and a second toner image is then formed and fused onto the second side of the media. In this duplexing process, the first toner image must pass through the heated and pressurized fusing nip a second time during fusing of the second image.
The fuser in many duplexing systems utilizes a heated roller and an unheated pressure roller. In the second stage of the duplexing process, the first image is contacted by the pressure roller during its second pass through the fusing nip. When the combined temperature of the pressure roller and the media rises above a particular level, the first toner image can partially or completely offset or transfer onto the pressure roller. In a worst case scenario, the media will wrap around the pressure roller and cause a media jam.
Toner particles are characterized by a cold offset temperature, below which the toner is not fused into the media. The cold offset temperature of a toner is determined by the composition of the toner and the parameters of the fusing system, such as the temperature, pressure, width of the fusing nip and the media speed through the nip. Toner particles are also characterized by a glass transition temperature, above which the toner becomes tacky and is prone to offset. For most color toners, the glass transition temperature is between about 65.degree. C. and about 70.degree. C.
For color toner, the proper fusing temperature in a typical fusing system is generally between about 140.degree. C. and about 170.degree. C. for many low volume photocopiers and laser printers. This fusing temperature is a combination of the temperature of the heated roller contacting the toner and the media temperature. In the case of duplexing, if the fusing system does not adequately control the combined temperature of the pressure roller and the media entering the fusing nip, this combined temperature can exceed the glass transition temperature of the toner being used. When this occurs, the first image can degloss and offset onto the pressure roller during the second pass of the media through the fusing nip.
In the prior art, the above problem has been addressed by employing various means of sensing and/or actively cooling the temperature of the pressure roller. An example of this approach is found in the U.S. Pat. No. 5,247,336 to Mills (the '336 patent). The '336 patent discloses a pressure roller that includes an internally mounted fan that draws air into and blows air out of the hollow interior of the pressure roller to control the pressure roller temperature.
While the components and system described in the '336 patent generally achieve the desired result, they require expensive and often bulky parts to be added to the printer or photocopier. This approach is especially impractical and disfavored in a compact and low cost desktop printer. Furthermore, even if the pressure roller is maintained at a temperature below the toner cold offset temperature, toner offset and degloss of the first side image may still occur during a long print or copying job, especially when the first side image has a high toner mass coverage. This can occur because the portion of the media path near the heated fusing nip is heated substantially by both fusing rollers, especially during sustained continuous printing or imaging. This radiated heat from the fusing nip can increase the temperature of the first side image by as much as 20.degree.-30.degree. C. or more above room temperature before the media enters the fusing nip. When the combination of the pressure roller temperature and the first side media temperature exceeds the glass transition temperature of the toner, toner offset and degloss of the first side image may still occur even though the pressure roller temperature is below the toner cold offset temperature.
The present invention seeks to overcome the shortcomings of the prior art by providing a simple and low cost apparatus and method for controlling media temperature to prevent offset and degloss of the first side image during duplexing. Media supports are provided upstream from the fusing nip. The media supports include cooling gaps between adjacent supports to minimize heating of the media prior to the fusing nip. An insulating channel is provided beneath the media supports to inhibit heat transfer to the media supports. One or more insulating plates may also be provided to inhibit heat transfer from the fusing rollers to the insulating channel and the media supports.